1. Field of the Invention
The invention relates in general to demodulators, and more specifically, to demodulators for demodulating an alternating signal modulated by frequency or time.
2. Description of the Prior Art
U.S. Pat. No. 4,275,429, which is assigned to the same assignee as the present application, discloses a new and improved solid state protective relay for protecting high-voltage, three-phase alternating current transmission lines. This relay provides the functions of an electro-mechanical pilot-wire protective relay, without the necessity of having a continuous, metallic conductor connected between the points to be compared. Thus, this relay, may be used with many different types of communication channels, including a dedicated, uninterruptible Bell Telephone system 3002 channel. While the 3002 channel is an open channel, it is not a continuous metallic circuit, as it may include amplifiers and switchboards in the circuit.
The modem used with this protective relay for the communication link between the points of the transmission line to be monitored and protected, must be able to operate within the constraints of the 3002 channel. The attenuation and envelope delay versus frequency, which will be known for each form of communication link which may be used with the relay, is a minimum in the 3002 channel in a narrow band of approximately 300 Hz around a center frequency of about 1.7 KHz. Thus, it would be desirable to be able to use a carrier frequency in this range when a 3002 channel is used.
A carrier frequency of about 1.7 KHz is a relatively low frequency in the art of protective relaying. Much higher carrier frequencies are normally used because it is desirable to be able to provide a trip signal in response to a fault in the protected line section with as little time delay as possible, such as about 1.5 cycles maximum. Thus, the modem, when operating with a carrier of about 1.7 KHz, cannot afford the time delay associated with filters in the demodulator for removing the carrier. For these reasons, the modem used in a preferred embodiment of the protective relay of U.S. Pat. No. 4,275,429 uses pulse period modulation (PPM) which, in addition to its accuracy and reliability, reduces the filtering requirements in the demodulator.
Co-pending application Ser. No. 239,917, filed Mar. 3, 1981, entitled "Pulse Modulator", now U.S. Pat. No. 4,380,746 which is assigned to the same assignee as the present application, discloses a new and improved pulse period modulator which may be used in the modem of the protective relay. This modulator is free of start-up and latch-up problems, and the carrier frequency is easily selectable in steps to suit the communication channel, from 110 KHz downward, including the desirable frequency range of 1.7 KHz.+-.200 Hz for the 3002 channel.
The demodulator may remove the carrier of the PPM signal without the use of large amounts of filtering, such as by using the principles of U.S. Pat. No. 3,614,641, which is assigned to the assignee of the present application. U.S. Pat. No. 3,614,641 teaches the use of two constant current sources, two ramp voltage capacitor circuits, and an auctioneering circuit. While the demodulator of U.S. Pat. No. 3,614,641 provides an excellent demodulation for frequency or time modulated signals, it would be desirable to provide a modulator which has fewer components, a lower manufacturing cost, and one which does not require the matching of circuit components, if these objectives can be achieved without incurring offsetting disadvantages.